NASSAU COUNTY, Fla. -- A First Coast News investigation has discovered hundreds of school buses in Florida that were sold with a part that the state says is not up to its specifications.

On March 24, 2014, Charlie Hood, the Director of School Transportation for the Florida Department of Education, wrote a letter to the head of Florida Transportation Systems, Inc., Chris Rustman, in Tampa.

The letter informed the longtime school bus provider that they sold nearly 640 buses to school districts across the state that contained a non-compliant air conditioning refrigerant hose.

Hood wrote that the buses "...were equipped with air conditioning refrigerant hoses that do not conform with the requirements of the contracts."

The hose is considered by experts to be a vital part of a school bus' air conditioning system. It often runs the length of the entire bus in a metal casing and helps keep students from overheating.

All of the buses were manufactured by Blue Bird and sold between 2010 and 2014. On the First Coast, Putnam, Nassau and Flagler Counties are listed by the state as purchasing school districts.

An attachment to Hood's letter shows Putnam owns 11 of the buses with the non-compliant part, Nassau owns 10 and Flagler owns two. Four of the buses in Nassau are new to the county and have not received their license plates or registration tags.

First Coast News took the information it uncovered to David Buchanan, Transportation Director for the Nassau County School Board.

He believes there was an unintentional mix-up in the contract and bidding process between all of the parties involved.

"What's on the bid should be on the bus. It just didn't happen. I understand there was a misunderstanding," he said.

Buchanan said while hose failures on school bus air conditioners are common, his district has had no reported issues with the non-compliant hoses.

"We have compliant hoses that have failed in the last three or four years. These non-compliant ones, none have failed so far," he told FCN reporter Jacob Long.

St. Johns County Public Schools faced a major issue with a school bus by the same manufacturer, Blue Bird, last fall. A coolant line burst and sent a couple of high school students to the hospital with reported burns on their skin.

District transportation officials have since told FCN the issue was thoroughly investigated and repaired on all of its buses. They also stressed the coolant line is separate from the refrigerant line and carries out a different function.

Still, the state is now requiring Florida Transportation Services, Inc. to bring the hundreds of buses with non-compliant hoses into line with its specifications.

The company must extend its warranty on the buses for another year or offer the affected school districts an opportunity to have the hoses repaired.

Buchanan said Nassau County will likely opt for the warranty. He said the district cannot afford to have so many buses out of order.

"It really is just a tiny technical issue," he called it.

State education officials said the incorrect hoses pose no threat to students. They said a different school bus vendor alerted them to the issue.

Erin Lake, the Marketing and Communications Manager for Blue Bird, said the once non-compliant hose is now considered the state standard and accepted for school bus air conditioning systems.

She said that change was in place at the time of FCN's investigation and that the air conditioning systems were an aftermarket installation. Blue Bird did not make or install the once incorrect hoses on hundreds of buses before they became standard.